Re: [PATCH] pwm: core: Permit unset period when applying configuration of disabled PWMs

[Thierry Reding <thierry.reding@gmail.com>]

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On Fri, May 12, 2023 at 02:20:12PM +0200, Marek Vasut wrote:
> On 5/12/23 08:22, Uwe Kleine-König wrote:
> > Hello Marek,
> > 
> > On Thu, May 11, 2023 at 08:18:53PM +0200, Marek Vasut wrote:
> > > In case the PWM is not enabled, the period can still be left unconfigured,
> > > i.e. zero . Currently the pwm_class_apply_state() errors out in such a case.
> > > This e.g. makes suspend fail on systems where pwmchip has been exported via
> > > sysfs interface, but left unconfigured before suspend occurred.
> > > 
> > > Failing case:
> > > "
> > > $ echo 1 > /sys/class/pwm/pwmchip4/export
> > > $ echo mem > /sys/power/state
> > > ...
> > > pwm pwmchip4: PM: dpm_run_callback(): pwm_class_suspend+0x1/0xa8 returns -22
> > > pwm pwmchip4: PM: failed to suspend: error -22
> > > PM: Some devices failed to suspend, or early wake event detected
> > > "
> > > 
> > > Working case:
> > > "
> > > $ echo 1 > /sys/class/pwm/pwmchip4/export
> > > $ echo 100 > /sys/class/pwm/pwmchip4/pwm1/period
> > > $ echo 10 > /sys/class/pwm/pwmchip4/pwm1/duty_cycle
> > > $ echo mem > /sys/power/state
> > > ...
> > > "
> > > 
> > > Permit unset period in pwm_class_apply_state() in case the PWM is disabled
> > > to fix this issue.
> > > 
> > > Fixes: ef2bf4997f7d ("pwm: Improve args checking in pwm_apply_state()")
> > > Signed-off-by: Marek Vasut <marex@denx.de>
> > > ---
> > > Cc: Brian Norris <briannorris@chromium.org>
> > > Cc: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
> > > Cc: Geert Uytterhoeven <geert+renesas@glider.be>
> > > Cc: Thierry Reding <thierry.reding@gmail.com>
> > > Cc: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
> > > Cc: linux-pwm@vger.kernel.org
> > > ---
> > >   drivers/pwm/core.c | 4 ++--
> > >   1 file changed, 2 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/drivers/pwm/core.c b/drivers/pwm/core.c
> > > index 3dacceaef4a9b..87252b70f1c81 100644
> > > --- a/drivers/pwm/core.c
> > > +++ b/drivers/pwm/core.c
> > > @@ -510,8 +510,8 @@ int pwm_apply_state(struct pwm_device *pwm, const struct pwm_state *state)
> > >   	 */
> > >   	might_sleep();
> > > -	if (!pwm || !state || !state->period ||
> > > -	    state->duty_cycle > state->period)
> > > +	if (!pwm || !state || (state->enabled &&
> > > +	    (!state->period || state->duty_cycle > state->period)))
> > 
> > While I tend to agree that the checks about period and duty_cycle are
> > (somewhat) irrelevant if enabled == false, I fear we'd break assumptions
> > here as some drivers configure duty_cycle/period in hardware even with
> > .enabled == false, and so proably rely on period > 0 and duty_cycle <=
> > period.
> > 
> > Another approach would be to skip pwm_apply_state() in
> > pwm_class_suspend() if the state is already disabled. That one sounds
> > safer.
> 
> I am not convinced that would be identical behavior.
> 
> If we skip apply_state call on PWMs exported via sysfs interface which are
> enabled=false , then the drivers wouldn't have their apply_state callback
> called to disable the PWM before suspend ... I think ... which seems like a
> problem to me ?

If a PWM exported via sysfs has enabled=false, then it should already be
disabled, so calling pwm_apply_state() on them to disable them should be
a no-op.

Thierry

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